1. Field of Technology
The invention relates generally to devices and methods to fractionate particulate matter into coarse and fine fractions. Applications include aerosol sampling (e.g., removal of large-sized debris from aerosols that are to be analyzed for biological materials) and powder processing (e.g., exclusion of large particles from pharmaceutical powders that are used for inhalation therapy).
2. Background of the Invention
A typical ambient aerosol sampling system includes a pre-separator that is designed to exclude relatively large particles from an aerosol comprising a spectrum of particle sizes. In some applications, the performance of the pre-separator is intended to mimic the human respiratory system by precluding transmission of larger-sized particles through the device. In other applications the pre-separator is used to strip particles that could confound post-separation analyses or could foul near-real time detection systems. For example, in the context of near-real-time bioaerosol sampling, it is generally necessary to strip large pollen particles from the distribution if the aerosol is to be analyzed for fluorescent characteristics.
Most conventional large particle fractionators are inertial separation devices, the simplest type being the classical inertial impactor. Inertial impactors are devices widely used for the sampling and size-selective collection of aerosol particles. The principal of operation for inertial impactors is that an aerosol stream is accelerated in a nozzle and impinges upon a collection surface, which is separated from the nozzle by an air gap. Particles in the aerosol stream having sufficiently high inertia will impact upon the collection surface while the other particles will follow the airflow out of the impaction region. For an impactor used as a pre-separator, the cutpoint particle size is usually defined as the aerodynamic particle diameter (AD) for which 50% of the cutpoint particles flow out of the impaction region with the air. This aerosol is referred to as the fine fraction. The ideal efficiency curve, which is the transmission efficiency as a function of particle size, is a unit step function at the cutpoint size. That is, all particles below the cutpoint size leave with the air in the fine fraction, while all particles above the cutpoint size are deposited on the collection surface or, inadvertently, on other internal surfaces.
In a conventional impactor-type pre-separator, to reduce undesirable large particle carryover due to particle rebound or re-entrainment from the collection surface, the particle collection surface is often coated with a layer of oil or grease that helps retain such larger particles. However, when the concentration of larger particles in the aerosol is relatively high, an oiled or greased surface may not provide sufficient large particle retention because the rate of dust accumulation on the surface may occur at a faster rate than the oil can be transported through the dust layer to the surface. An additional problem with oil- or grease-coated collection surfaces it that fibrous particles in the collected deposits will cause inadvertent collection of particles from the fine fraction. Frequent maintenance is typically needed to clean the collection surfaces of real impactors to reduce the collection of fine-fraction particles by previously deposited fibrous materials and provide sufficient coarse particle retention on the collection plate.
As an alternative to classical inertial impactors, virtual impactors may be used to fractionate an aerosol into coarse and fine fractions. If a suitably shaped collection surface, at the location where the particles would impact in a real impactor, has an opening through which large particles may pass, and if a small amount of transport air is used to convey the large particles away from the opening, a “virtual impactor” can be created. Coarse particles enter the opening with the minor flow while the fine particle flow, i.e. the major flow, is separated therefrom. Thus, particles with a size above the cutpoint (together with the small particles in the transport air) are conducted away by the minor flow stream of gas leaving a size distribution of the fine particle flow that is scalped of large particles. In this way, virtual impactors tend to minimize large particle carryover relative to the undesirable large particle carryover that can result with the use of real impactors.
Although the present invention is primarily focused on the fine particle fraction in the major flow, a virtual impactor concentrates the coarse particle fraction and in many applications, the coarse fraction is of interest. The coarse particle fraction leaves the virtual impactor with an air flow (minor flow) that is reduced in volume compared with the air flow that approached the “virtual collection surface.”
Typically, the slope of the transmission efficiency curve of a virtual impactor is not as steep as that of a real impactor, i.e., the transmission efficiency of the real impactor more closely approximates that of an ideal transmission efficiency curve. On the other hand, the large-particle fractionation characteristics of the human respiratory system are more closely approximated by a virtual impactor than by a real impactor.
Virtual impactor geometry tends to be complex. For example, correct alignment of the acceleration nozzle with respect to the nozzle that receives the coarse particles (minor flow) and the nozzle that receives the fine particles (major flow) is critical to flow stability and performance, but difficult to consistently achieve. Consequently, most large particle fractionators still employ some form of a real impactor.
Accordingly, there remains a need in the art for improved devices and methods for fractionating aerosol particles in applications such as bioaerosol sampling and production of pharmaceutical powders.